Segment Information - Schedule of Segment Reporting Information by Segment (Detail) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2022 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2022 USD ($)	Jun. 30, 2021 USD ($)	Dec. 31, 2021 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments				2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 139,470	$ 111,082	$ 362,350	$ 302,268
Operating Income
Total operating income		26,469	22,578	81,334	74,175
Other expense, net		2,584	1,453	3,498	1,828
Interest		5,825	5,054	11,164	10,159
Income Before Income Taxes		23,228	18,977	73,668	65,844
Income Taxes		6,177	5,164	19,683	17,565
Net Income		17,051	13,813	53,985	48,279
Identifiable Assets
Total identifiable assets		2,117,342		2,117,342		$ 2,114,869
Regulated Energy [Member]
Operating Income
Total operating income		25,841	22,760	60,539	55,466
Identifiable Assets
Total identifiable assets		1,637,003		1,637,003		1,629,191
Unregulated Energy [Member]
Operating Income
Total operating income		560	(465)	20,613	18,575
Identifiable Assets
Total identifiable assets		447,209		447,209		439,114
Other [Member]
Identifiable Assets
Total identifiable assets		33,130		33,130		$ 46,564
Other and eliminations [Member]
Operating Income
Total operating income		68	283	182	134
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		139,470	111,082	362,350	302,268
Operating Revenues, Unaffiliated Customers [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		91,830	80,374	215,888	201,095
Operating Revenues, Unaffiliated Customers [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		47,640	30,708	146,462	101,173
Intersegment Revenues [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	6,271	4,734	12,706	9,635
Intersegment Revenues [Member] | Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	363	536	4,196	1,012
Intersegment Revenues [Member] | Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	5,823	4,065	7,819	8,358
Intersegment Revenues [Member] | Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[1]	85	133	691	265
Retained Earnings [Member]
Operating Income
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent		17,051	13,813	53,985	48,279
Net Income		$ 17,051	$ 13,813	$ 53,985	$ 48,279

[1] All significant intersegment revenues are billed at market rates and have been eliminated from consolidated operating revenues.